The influence of minoxidil on cardiovascular hemodynamics and on regional myocardial blood flow in beagle dogs was investigated. Minoxidil caused increases in mean right atrial and left ventricular end-diastolic pressures. Systemic and pulmonary vascular resistances were reduced; cardiac outputs were increased. Left ventricular stroke work and the tension time index were unchanged by minoxidil administration. Regional myocardial blood flow was measured using radioactive microspheres. Minoxidil increased coronary blood flow to all regions of the heart except to the left ventricular papillary muscles, in which, it was unchanged in comparison to blood flow in untreated animals. Minoxidil increased blood flow to left ventricular subendocardial tissue but significantly less than corresponding areas of subepicardial tissue, reducing the subendocardial/subepicardial blood flow ratio.